Regesta Pontificum Romanorum In the near future there will be a regest for every papal contact prior to the year 1198.
As of now just 282 regesta of the Bohemia-Moravia (Friedrich-Alexander-Universität Erlangen-Nürnberg)

Pope Gregory VII Information Database "Information on where and in which collections/contexts the letters of pope Gregory VII were transmitted (Christian Schwaderer, Eberhard-Karls-Universität Tûbingen)

Corpus Juris Canonici (1582) (UCLA, CMRS)
A facsimile edition of the Decretals
of Gregory IX as they appeared with marginal commentary in the Corpus
canonicum glossatum promulgated by Gregory XIII in 1580.

Early English Laws (EEL) Bibliography [a collaboration between the Institute of Historical Research and King's College London, in the period between c. 600 and 1215, searchable by category, author and date of publication]